In the technical field of solar cells, silicon heterojunction technology can achieve very high efficiencies and has the potential to provide lower production costs in high volume manufacturing. In order to lower the costs and carbon output caused by fossil fuels in energy generation, attempts have been made to convert the production of energy to cleaner, cheaper alternative energy sources, such as solar power.
A solar cell uses the principles of photoelectricity to convert energy within the sun's light into electricity. The various types of solar cells may include differing p- and n-junctions, such as cells with single or multiple junctions. In solar cells with a single p-n junction, the stacks of p-doped and n-doped layers made of similar materials with equal band gaps are said to form a homojunction solar cell. Single-junction stacks with at least two layers of varying band gap material are called heterojunction solar cells.
When sunlight hits a solar cell, light absorbed near the p-n junction generates carriers. The electric field across the junction separates the carriers that have diffused into the p-n junction and in turn, produces an electric current in the solar cell which may be transferred to attached devices. The quality of a solar cell is measured by the energy conversion efficiency, i.e., the ratio between the converted power from the sunlight and the transferred power to an electrical circuit.
Silicon heterojunction (SHJ) solar cells can achieve higher efficiencies than homojunction cells because of an inherent band gap between an emitter layer of amorphous silicon (a-Si) and a base layer of crystalline silicon (c-Si) that acts as a barrier for minority carriers to reduce the recombination velocity at the cell surface otherwise seen in homojunction cells with dangling bonds. The thinner a-Si layer in a heterojunction solar cell can passivate the surface of the c-Si base layer by repairing these dangling bonds and thus, maintaining the higher efficiency of the stack as compared with the homojunction solar cell. For these reasons, SHJ solar cells may provide greater efficiencies with higher open-circuit voltage (VOC) and larger short-circuit current (JSC).
In current SHJ cells, doped a-Si films are deposited in layers on both sides of the base layer (or wafer) to form p- and n-type carrier collectors. Usually a transparent conductive oxide (TCO) layer is formed on top of the doped a-Si films to achieve lateral conductivity and good ohmic contact with metal electrodes. On the front (or top) sides of cells, TCO layers also serve as an antireflective coating, while at the rear (or back) side, TCO layers can be optimized in conjunction with a corresponding metal layer to form an infrared reflector.
Optimization of TCO layers in SHJ solar cells is a subject of multiple tradeoffs between optical, recombination, and series resistance gains and losses. The generally desired TCO properties include, but are not limited to: (1) a refractive index close to 2.0 for serving as an antireflective coating in front of the silicon wafer; (2) transparency in the entire ‘silicon solar’ range (from about 350 to 1200 nm) for the front layer, transparency from approximately 800 to 1200 nm for avoiding parasitic absorption of infrared (IR) light for the rear layer; (3) a necessary lateral conductivity for front layers usually in the range of about 50 to 100 Ω/□, which is determined by the front grid design; (4) good ohmic contact with the corresponding metal electrodes and doped a-Si films for forming Schottky barriers; (5) deposition without damage to the underlying a-Si film and without inducing change in the a-Si/crystalline Si (c-Si) heterojunction, which could limit the performance of the SHJ solar cell; and (6) an absence of reliability issues for preventing degradation and failures in the field.
It is often the case that material properties favorable for certain design requirements are harmful to the others. As a result of the tradeoffs between the listed requirements, most SHJ cells (including record cells) usually have lower JSC compared to diffused junction cells (for most reported cells <40 mA/cm2) with a characteristic low response for the external quantum efficiency (EQE) in 350-600 nm range. These lower values are partly due to absorption in the a-Si top layers, but also due to the absorption in the front TCO antireflective coating layers. Additionally, if not optimized, the rear TCO/metal layer stack can cause significant absorption in the IR portion of the spectrum leading to additional JSC losses.
Therefore, a system and method for optimizing the design of Si heterojunction solar cell layer stacks, such that optical response is increased and operation is enhanced, that will not include significant losses in resistance and other properties at a low cost of production and that will alleviate other identified issues, is highly desirable.